Does anyone has deeper knowledge regarding BIS, and why it should not be possible to use imap instead of pop ?

I have a client, who has BIS.
I did set up a new Blackberry mail account at the providers Web-page (instantemail.t-online.de)
If I use the first option, and only enter my email address and password. POP3 is then automatically used and works ok
But imap is better, and maybe then I have a copy of sent mail in my sent items box. So I try to use imap:
The only way to get a litte more control of the setup process is to enter an invalid email adress. Then you can specify a server, username etc. etc.
The mailserver in question answers imap on port 143 and 993 (ssl).
However the BIS setup assistant refuses and tells me that there is a problem and it cannot continue.

From past BIS versions, I know, that there was an option to enter everything manually including specifiying IPAM or POP. But this new (?) assistant does not allow to specify the protocol to use for the mailbox access.
I called the provider (t-mobile), and, they again walked me through all steps, but, we finally gave up.

Does anyone know a workaround here or a technical explanation why the BIS setup assistant may refuse to accept IMAP ?